In typical communication systems, oscillators, e.g., voltage-controlled oscillators (“VCOs”) and digitally-controlled oscillators (“DCOs”), are used in applications such as clock and data-recovery circuits for serial data communications, radio frequency communications, clock distribution, and integrated frequency synthesizers. A DCO provides an output carrier signal, the frequency of which is determined by a digital control word that tracks (or tunes to) environmental variations. For example, the frequency of a DCO may vary as much as 3-4% (or 30,000-40,000 ppm) due to changes in ambient temperature. To avoid large glitches during frequency adjustments, the frequency change per control bit (ΔfLSB) should preferably be small, e.g. on the order of 120 ppm.
Common DCO implementations include an array of tuning elements or units addressed by a digital control word. In one implementation, a separate control signal is used to address each array element, which, for high-frequency and large-tuning-bandwidth DCOs having a large number of tuning elements, results in an unacceptably large wiring layout. On the other hand, a DCO with a small number of tuning elements, along with a preferably small ΔfLSB, generates frequencies covering a small runtime tuning bandwidth which may be insufficient to track the frequency variations mentioned above. Another DCO implementation includes a two-dimensional array of elements and a decoder logic circuit to address the elements. In this implementation, the decoder logic circuit may employ a “thermometer decoding” approach to activate array elements using sets of row and column wires, thereby reducing the wiring requirements to approximately 19% of the wires required for the above-mentioned approach in which each element is addressed by its own wire. This array implementation, however, uses elements with larger ΔfLSB (e.g., approximately equal to 400 ppm), resulting in poorer frequency resolution and large frequency glitches. These drawbacks may cause an already-locked carrier frequency to be lost, ultimately resulting in erroneous data sampling and decoding.
Therefore, a need exists for a compact and efficient DCO with a wide dynamic range, suitable for tracking temperature-induced variations in frequency, and which also locks and tracks a carrier frequency reliably.